Because no sane person would pass up the opportunity to be the first woman President of the United States.
I'm playing devil's advocate. Hillary is certainly keeping the door open, and making the moves to have a successful campaign if she does run, but I believe her when she says she hasn't made up her mind.
One difference is that Hillary Clinton has sought the presidency before.
People who come close to the White House often make another go at it if there's an opportunity.
Although there are exceptions. See Al Gore in 2004.
It does remain possible that Clinton won't run. Maybe there's a health issue, or whatever.
